The experiments of [1]} and [2]} were conducted in nominally “clean” channels without added surfactant, and where contaminant surfactant concentrations were typical of environmental values.
In contrast, [3]} added surfactant in their experiments, finding barely measurable increases in drag.
As noted by [1]}, the counterintuitive result reported by [3]} was most likely due to the fact that contaminant surfactant was already present in their experiment.
Contaminant surfactants are often naturally present in most engineered systems, due to general manufacturing conditions and materials.
For instance, microfluidic experimental devices are often made of polydimethylsiloxane (PDMS), which is known to lead to surfactant effects [6]}.
In nature, surfactant traces have been measured in sea water [7]}, rivers, estuaries and fog [8]}, [9]}.
